A measurement process similar to the type described above is discussed in a brochure entitled "Transmissions-Photometer", published by Pier-Electronic GmbH, Nassaustrasse 35, D-6238 Hofheim-Wallau (Germany). By means of the process described therein, low levels of optical absorption are measured by a joint analog acquisition of measurement and reference signals detected in alternation at a high frequency of alternation, in the form of their quotient. The consideration of the natural intensity difference between the measurement light beam and the reference light beam is accomplished by the mathematical inclusion of an intensity quotient of the two light beams without any absorption by the material to be tested. The optical absorptions are therefore determined indirectly by this process, since the quotient determined is not a direct measurement of the optical absorption. The optical absorption is calculated only from the difference between the number 1 and the quotient. However, this conversion includes the possibility of increased error propagation. With low levels of absorption, in particular, relatively severe changes in the optical absorption (e.g., from 1% to 2%; i.e., by a relative 100%) result in only small changes in the quotient (e.g., from 0.99 to 0.98; i.e., by a relative 1%). Even small measurement errors, or rounding off for digitization, thus cause a much greater inaccuracy in the determination of the optical absorption. With low levels of optical absorption in particular, the error can therefore automatically approach or exceed the order of magnitude of the absorption being measured.